The present invention relates to a method of molding toothbrushes and to an apparatus for performing the method.
Molding of toothbrushes with tufts of fibre or bristles incorporated in the head portion of the brush body during the injection step involves a specific problem. One of the relatively movable mold parts has a brush head defining cavity portion with a plurality of holes for partial penetration into the cavity portion of a like number of fibre tufts. Due to the viscosity of the heated plasticized molding material and to the pressure within the cavity during injection, leakage is likely to occur through the holes in the brush head defining cavity portion. Many attempts have been made to avoid such leakage. One of such attempts, as disclosed in EP 0 676 268 A1, was to form a knob of molten material at each tuft end, and holding such knob against the edge defining the corresponding hole in the brush head defining cavity portion during injection of the plasticized material. Another attempt (DE 38 32 520 A1) was to introduce pins coaxially into each tuft to urge the bristles radially outwardly against the wall of the hole. However, none of these attempts was successful in reliably avoiding any leakage.
The present invention proceeds from the idea that the only manner to avoid such leakage is a precise and consistent control of pressure within the mold cavity during injection. In accordance with the invention, a mold with relatively movable mold parts defining at least one molding cavity corresponding in shape to a toothbrush body is used. One of the mold parts has a brush head defining cavity portion with a plurality of holes for partial penetration into the cavity portion of a like number of fibre tufts. The inventive method comprises the steps of providing a supply of plasticized molding material, injecting the plasticized material under pressure into the cavity, sensing pressure within the cavity during the injecting step, and stopping injection when the pressure within the cavity exceeds a predetermined pressure value. Although usual molds are equipped with pressure sensing and pressure adjusting means, the invention proposes to sense pressure during the injection step in the very cavity, and in the preferred embodiment, at one end of the elongate cavity which corresponds to the brush head portion. In the preferred embodiment, the injection of molding material is performed through a narrow injection channel which opens into the cavity at an end opposite the end where pressure is sensed, and stopping of injection is carried out by suddenly blocking material flow through the channel at a location close to the cavity.
In the preferred embodiment of the method, a plurality of toothbrushes are simultaneously molded in a plurality of cavities and pressure is sensed in each cavity. Stopping of injection in each individual cavity is controlled by pressure sensed in that cavity. Although a mold with a plurality of like cavities is usually provided with a balanced manifold system, identical amounts of pressure in each cavity cannot be achieved due to manufacture tolerances and fluctuations of the molding parameters such as viscosity and temperature. By sensing pressure within each cavity and individually controlling a valve member associated with each cavity, in dependence upon pressure sensed in that cavity, identical pressure conditions can be achieved throughout the cavities.
The predetermined pressure level at which supply of molding material of the cavities is suddenly stopped must be selected below a value where leakage starts to occur. The appropriate pressure level will depend on the nature of the molding material and on the characteristics of the mold. It can be determined empirically.
The invention is not limited to methods wherein the brush body is molded from a single molding material. Rather, a first molding material can be injected in a first injection step forming a frame brush incorporating the fibre tufts, and at least one further molding material component is injected in a second molding step to complete the toothbrush.
A precise pressure control as described in this invention may only be needed during the first injection step. However, a second control system may be needed if the second component also imbeds (part of) the tufted portion of the brush or if the second component gets into the vicinity thereof (eg. for brushes featuring second component in the brushhead).
The invention also provides an apparatus for performing the method. The apparatus comprises a mold with relatively movable mold parts defining at least one molding cavity corresponding in shape to a toothbrush body. The cavity has a brush head defining cavity portion with a plurality of holes for partial penetration into the cavity portion of a like number of fibre tufts. The apparatus further comprises a plasticizing and pressurizing supply unit. A narrow supply channel connects the cavity with said supply unit. A pressure sensor is associated with the cavity. A valve member adapted to shut off the supply channel is arranged adjacent the cavity. A control unit controls the valve member in dependence upon pressure sensed by the pressure sensor within the cavity. In the preferred embodiment of the apparatus, the mold comprises a plurality of like cavities, and the supply unit comprises a manifold with a plurality of supply channels each connected to one of the cavities, and each supply channel has an associated valve member. Preferably, each valve member is controlled individually by the control unit in dependance upon pressure sensed in the associated cavity.